To a large extent decanter centrifuges are used for dewatering sludge from industrial processes and from waste water treatment, where large volumes of sludge are obtained, e,g. after chemical and/or biological precipitation of waste water. The sludge may contain large amounts of chemicals, such as alkalis, particularly within mining and cellulose industries, which should not be retained in the sludge, but which instead should follow the reject and be returned to the process. For final drying and for treatment of the sludge it is required that the maximum content of substances that can be washed out is eliminated by means of washing of the sludge for reduction of the costs in the subsequent treatment.
A decanter centrifuge incorporates a drum with a cylindrical and a conical part. The drum is driven by means of a motor. The rotational speed of the drum is normally about 1.500 to 4.000 rpm. Inside the drum is provided a helical screw, which also is rotatably arranged, either via the drum motor or via a separate motor. The blades of the helical screw hereby form an angle less than 90.degree. against the longitudinal axis of the centrifuge in a direction towards the front end of the centrifuge, which means that the blades of the helical screw are arranged obliquely in the forward transport direction. At rotation of the drum a sludge containing the sludge portion of the water is thrown against the circumference of the drum and its water portion is pressed out of the sludge inwardly towards the centre of the drum. The water thereupon can be eliminated through the reject outlet. By rotating the drum and the helical screw at different rotational speeds a transport effect is obtained thus that the sludge is fed towards one end of the drum, the conical one, for being taken out there. If the drum and the screw have the same rotational speed, no feed is obtained. The helical screw can be individually designed depending on the type of sludge.
Occurring types of sludge are waste water treatment sludge, which has a very heterogenous composition, and on the other hand industrial sludge from e.g. the cellulose industry, where the sludge has a more homogenous composition. In all cases the sludge most often contains water retaining material, such as cellulose fibres, metallic hydroxide flocs and other things. The requirement however is always to obtain such a high final TS-number (content of total solids) as possible and in certain cases to obtain such a clean sludge as possible.